Same as the other guy said. You'll catch each other's eye and he'll smirk or smile. Sometimes he'll add a slight nod of acknowledgement or courtesy. And women on instinct know what the smile (or smirk) looks like. It's not some "I'd like to smear mustard all over your feet while I masturbate to Barry Mannilow and whip you with a dirty diaper" look. It'll be more like an approving "I like what I see at a glance" type smile.
I'd say there's a few but one I think that is popular is one eyebrow raised, one lowed and a smirk-type smile with the side of the smile is lifted on the same side the eyebrow is lowed. It's like a "curios/mysterious/sexy look."
then there is pretty good chance he likes you and is interested. =)
i agree with chocolate-lover...
the look is longer than usual and very focused, meaning he doesn't blink or anything, and his mouth might be open and eyebrows raised..bc he's just in awe from seeing you and you being so beautiful...
its like when you see fireworks...ur face lights up and mouth wide open eyes big and brows raised...because you like what you see...same thing with attraction...if a guy or grl like each other, theyll naturallly be in awe to see one another
If you notice him raise his eyebrows when he first glances at you.